A conventional alarm system used indoors, outdoors or in a vehicle is designed only with a buzzing function. Such alarm system emits loud noise when activated while failing to create an effective burglarproof function.
As shown in FIG. 1, the aforesaid conventional alarm system includes a low frequency oscillator 10, an audio oscillator 11, a Darlington amplifier 12, and a speaker 13, wherein the oscillating signal of the audio oscillator is modulated by the low frequency oscillator and then amplified by the Darlington amplifier. The alarm system is us usually provided with a 1.2 V power source so that the power is quite limited and the magnitude of the emitted sound cannot be stably adjusted. Moreover, the emitted sound is constant without variation and cannot be recorded. Therefore, the range of use is restricted.